1. Field of the Invention
The present invention relates generally to the field of direct oxidation fuel cells and, more particularly, to a flow through gas separator for separating out and directing the flow of gases produced in the reactions in the fuel cell.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel-processing is expensive and requires significant volume, reformer based systems are presently limted to to comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically-conductive but, electronically non-conductive membrane (PCM). Typically, a catalyst which enables direct oxidation of the fuel on the anode is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Protons (from hydrogen found in the fuel and water molecules found on the anodic face of the reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and Oxygen molecules involved in the cathodic reaction and travel through a load, providing electrical power.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, methanol in an aqueous solution is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two fundamental reactions that occur in a DMFC which allow a DMFC system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water.
As noted, the DMFC produces carbon dioxide as a result of the reaction at the anode. This carbon dioxide is separated from the remaining methanol fuel mixture before such fuel is re-circulated. Carbon dioxide may be treated as waste, and removed from the system, or used to perform work within the DMFC system before it is vented or otherwise removed. For example, and not by way of limitation, the carbon dioxide gas can be used to passively pump liquid methanol into the fuel cell. This is disclosed in U.S. patent application Ser. No. 09/717,754, filed on Nov. 21, 2000, for a PASSIVELY PUMPED LIQUID FEED FUEL CELL SYSTEM, which is commonly owned by the assignee of the present invention, and which is incorporated by reference herein in its entirety. Another method of utilizing the carbon dioxide is described in U.S. patent application Ser. No. 09/837,831, filed on Apr. 18, 2001, for a METHOD AND APPARATUS FOR CO2-DRIVEN AIR MANAGEMENT FOR A DIRECT OXIDATION FUEL CELL SYSTEM, which discloses a method of using carbon dioxide to actively draw more air to the cathode face of the protonically conductive membrane, thus ensuring that sufficient oxygen is available to continue the cathodic reaction as necessary, and to minimize energy loss from Oxygen transportation.
Fuel cells and fuel cell systems have been the subject of intensified recent development because of their ability to efficiently convert the energy in carbonaceous fuels into electric power while emitting comparatively few environmentally harmful substances. The adaptation of fuel cell systems to mobile uses, however, is not straight-forward because of the technical difficulties associated with reforming most carbonaceous fuels in a simple, cost-effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for substantially pure hydrogen (which is a gas under the relevant operating conditions), presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy absorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol or an aqueous methanol solution (and to a lesser extent, ethanol, propane, butane and other carbonaceous liquids or aqueous solutions thereof). Efforts to develop DMFC systems commercially have increased over the past several years.
In particular DMFCs are being developed for commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell, and the components must be fabricated using materials that not only optimize the electricity-generating reactions, but which are also cost effective. Furthermore, the manufacturing process associated with those materials should not be prohibitive in terms of labor intensity cost.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and a direct methanol fuel cell (“fuel cell”). The fuel cell typically consists of a housing, anode and cathode flow field plates, anode and cathode diffusion layers, and a membrane electrode assembly (“MEA”) disposed within the housing.
A typical MEA includes a centrally disposed protonically conductive, electronically non-conductive membrane (“PCM”). The membrane is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer functions to evenly distribute the liquid fuel across the anode in the case of the fuel or the gaseous oxygen across the cathode face of the PCM in the case of the oxygen. In addition, flow field plates are often placed on the surface of the diffusion layers which are not in contact with the coated PCM. The flow field plates function to provide mass transport of the reactants and by products of the electrochemical reactions, and act to collect and conduct electrons through the load.
As noted, the DMFC produces carbon dioxide as a result of the anodic reaction at the anode. If carbon dioxide is allowed to accumulate, the pressure within the DMFC system may cause the system to fail mechanically. It is well known in the art that anodically evolved carbon dioxide must be removed from the system. Alternatively, this carbon dioxide can be used to drive other aspects of the system. However, in order for it to be used, the carbon dioxide be separated from the remaining, unreacted methanol fuel mixture before such fuel is re-circulated.
It has been known to provide a gas separator to remove the CO2 from the anodic effluent of a liquid feed direct oxidation fuel cell. However, presently available gas separators suitable for use in DMFC systems are membrane-based or mechanical in nature, and may be difficult to incorporate into present designs or within desirable form factors. The membrane-based devices require pressures greater than those found within a DMFC system to operate effectively. Mechanical gas separators may only function effectively in a single orientation with respect to gravity, and thus may not be suitable for use in portable electronics and other electronic tools that need a certain level of orientation independence.
There remains a need, therefore, for a gas separator for use with a liquid feed, direct oxidation fuel cell that does not require high pressures for its operation, and that is orientation independent.
It is thus an object of the present invention to provide an apparatus that separates anodically-produced CO2 from anodic effluent in a direct oxidation fuel cell. It is another object of the invention to provide a gas separation device that allows for capture of the anodically-separated CO2 so that it can be used for other purposes within in the cell. It is a further object of the invention to allow un-reacted aqueous methanol, for example, to be re-circulated in the cell.
It is yet a further object of the invention that we provide a fuel cell, having a gas separation device, that operates effectively independent of its physical orientation. It is a further object of the invention to provide a gas separation device that can operate without the necessity of high pressure conditions.